MacPherson struts are being employed in many present day and future vehicles. These struts comprise a combination wheel suspension strut and shock absorber which eliminates the requirement for an upper control arm. The elimination of the upper control arm provides an increase in space for the engine and the transmission or transaxle within the engine compartment of the vehicle. This additional space is especially advantageous for front wheel drive vehicles which include an Aeast-west≅ engine rather than the traditional Anorth-south≅ orientation for rear wheel drive vehicles.
A typical MacPherson strut includes the suspension spring positioned co-axially around the shock absorber. The spring extends from a bracket mounted on the outer tube of the shock absorber to a bracket that is integrated into the top mounting system for the MacPherson strut. The suspension spring can be a steel coil spring or the suspension spring can be an air spring.
When an air spring is used with the MacPherson strut, the air spring is positioned coaxially over the shock absorber assembly and it typically employs an air spring which is rigidly fixed to the vehicle body. The air spring is designed to be of a sufficient diameter to accommodate articulation of the shock absorber through the normal suspension jounce and rebound motion of the vehicle. The internal diameter of the air spring must therefore be large enough so as not to pinch the air spring bellows between the shock absorber body and the outer canister of the air spring, as the suspension is articulated. To reduce the required diameter of the air spring body, the upper portion of the air spring can be mounted to a spherical bearing. The spherical bearing will allow conical motion so that the air spring can articulate coaxially with the shock absorber. This coaxial movement reduces the diameter of the air spring because the outer canister of the air spring moves with the shock absorber rather than the shock absorber moving towards a stationary outer canister. In order to prevent air leakage of the air spring through the sliding surfaces of the spherical bearing, the sliding elements of the bearing are typically positioned external to the pressure chamber of the air spring.
While the addition of a spherical bearing allowing conical motion so that the air spring articulates coaxially with the shock absorber allows the diameter of the air spring to be reduced, the packaging space for both the upper shock isolator and the separate spherical bearing assembly is still generally unacceptably large. The spherical bearing assembly can be located axially above the air spring and the upper shock isolator assembly similar to a ball joint, but this configuration reduces the available length of the shock absorber which has the undesirable effect of reducing the allowable suspension travel. If the spherical bearing assembly is located external to the air spring canister and is generally in the same plane as the shock absorber isolator, the overall diameter of the upper mount assembly becomes unacceptably large.
The continued development of the MacPherson strut suspension systems includes the design and development of mounting systems which optimize the space required for the mounting system.